The turnover of nuclear RNA in sea urchin embryos is being studied. The RNA in the form of nucleoproteins is extracted in high salt and purified by differential centrifugation. The susceptibility of these ribonucleoproteins to nucleases present in various cell fractions will be assessed. Particular attention will be given to nuclear fractions containing nuclease activity as these enzymes may be involved in nuclear RNA turnover and processing. Nuclease activity will be measured by electrophoresing the RNA on denaturing gels and looking for small oligonucleotides plus any larger fractions that may be conserved for transport as messenger RNA. The latter will also be evaluated by hybridization to stage specific mRNA probes. We are also continuing a study of tRNA synthesis in cleavage and blastula embryos. We have evidence for the synthesis of several species of tRNA's at hatched blastula and want to determine (1) when other classes of tRNA are synthesized and (2) at what stage does tRNA synthesis commence and at what rate.